The primary disadvantage of sheet grippers for sheet-fed rotary printing presses is that the retaining surfaces cannot compensate for errors in parallelism caused by variations in operational, manufacturing and assembly parameters nor for the effect of the thickness of the material being printed. Since a sheet is retained non-positively, an error in parallelism will result in non-uniform gripping pressure over the area of the retaining surface. The non-uniform gripping pressure leads to an increased likelihood of slipping, especially when smooth paper is being printed at high press speeds, which, in turn, causes registration errors rendering the printed material unusable.